Currently within the electronics field, there is a significant demand for an insulating film which can be grown epitaxially and consistently on silicon. Such a film would greatly enhance the capabilities of silicon-based devices for operation at temperatures far exceeding the present limitations.
Boron nitride (BN) is a most interesting III-V compound from both the practical and scientific viewpoints. Boron nitride is characterized by three different crystal structures: hexagonal, wurtzite and cubic zincblende. The boron nitride phase having the cubic zincblende crystal structure is particularly useful since it is characterized by many desirable physical properties including high electrical resistivity and high thermal conductivity. In addition, the cubic zincblende boron nitride is relatively inert chemically. Further, cubic boron nitride films are very hard. Because of these properties, this cubic form of the boron nitride is potentially very useful for electronic devices, particularly for use at high temperatures.
A cubic form of boron nitride has been grown on silicon wafers by means of a laser ablation technique, as disclosed in U.S. patent application Ser. No. 07/446,758 to Gary L. Doll et al, entitled "Laser Deposition of Crystalline Boron Nitride Films", filed on Dec. 6, 1989, and assigned to the same assignee of this patent application. With this laser ablation method, single crystal cubic boron nitride films were epitaxially grown on a silicon substrate oriented along the [100] axis, such that the resulting cubic boron nitride films were in epitaxial registry with the underlying silicon substrate. Two epitaxial registries have been observed for cubic boron nitride on silicon. One epitaxy has the principle axis of a cubic boron nitride with a 0.362 nanometer lattice constant parallel to the crystallographic axes of the silicon, such that three cubic boron nitride lattices oerlay two silicon lattices. The other epitaxy has the [100] direction of a cubic boron nitride with a lattice constant of 0.384 nanometers notched to align with the [110] silicon axis. In this way, two cubic boron nitride lattices overlay one silicon lattice. Since fewer uncompensated silicon bonds exist in the second epitaxy, it is more energetically favorable than the first.
As stated above, the cubic boron nitride has many characteristics useful for high temperature electronic applications. However, in order to successfully grow the cubic boron nitride on the silicon substrates with the more energetically favorable epitaxy, the crystallographic lattice for the cubic boron nitride must expand to match the lattice constant of the underlying silicon. In particular, the cubic boron nitride films formed on the silicon substrate by the laser ablation method described above, are characterized by a crystallographic lattice constant of approximately 0.384 nanometers as compared to the lattice constant of approximately 0.362 nanometers for bulk cubic boron nitride powder. The lattice constant for the cubic boron nitride films formed by the laser ablation method is approximately 5 percent larger than the bulk material. Because of this lattice expansion, two cubic boron nitride unit cells can fit along the [110] silicon direction, so as to result in epitaxial registry between the silicon and cubic boron nitride.
Although this lattice expansion brings the cubic boron nitride into crystallographic registry with the underlying single crystal silicon lattice, a large dislocation energy is always associated with a lattice expansion of this magnitude. It is believed that this large dislocation energy may be accommodated by the presence of pinholes and internal stresses within the film. Without these mechanisms, or some other vehicle for accommodating this large dislocation energy, the cubic phase for boron nitride is energetically unfavorable.
However, these mechanisms present serious engineering problems within the electronic devices formed form these films. One problem is that there is a substantial electrical current leakage through the pinholes. This effect, as well as other less serious problems, if uncorrected makes the cubic boron nitride a less desirable insulator as compared to other materials, such as amorphous silicon dioxide for example.
Therefore, it would be desirable to alleviate these problems and provide an epitaxial insulator for silicon which has a lattice constant similar to the lattice constant of the underlying silicon, and which thereby avoids the shortcomings of the art. In particular, it would be desirable to provide an epitaxial insulator which is characterized by a lattice constant which is substantially equal to .sqroot.2/2 times the lattice constant of the underlying single crystal silicon substrate, so as to minimize or avoid the lattice mismatch and expansion of the epitaxially grown insulating film. An epitaxial film having the thermal and physical characteristics of cubic boron nitride, but with a matched lattice in epitaxial registry with the underlying silicon, would be an excellent candidate for high temperature electronic devices.